PCO2, partial pressure of carbon dioxide; PH2O, partial pressure of water; PN2, partial pressure of nitrogen; PO2, partial pressure of oxygen; PTOTAL, partial pressure of all parameters; R, respiratory quotient. *PN2 is increased in alveolar gas by 1% because R is normally less than 1.
†Ptotal is less in venous than in arterial blood because PO2 has decreased more than PCO2 has increased.
where PiO2 is the partial pressure of inspired O2, which is equal to the fraction of O2 (FiO2) multiplied by the barometric pressure (Pb) minus water vapor pressure (PH2O); PACO2 is the partial pressure of alveolar CO2; and R is the respiratory exchange ratio, or respiratory quotient. The respiratory quotient is the ratio of the amount of CO2 excreted (V̇ CO2) to the amount of O2 taken up (V̇ O2) by the lungs. This quotient is the amount of CO2 produced in relation to the amount of O2 consumed by metabolism and is dependent on caloric intake. The respiratory quotient varies between 0.7 and 1.0; it is 0.7 in states of exclusive fatty acid metabolism and 1.0 in states of exclusive carbohydrate metabolism. Under normal dietary conditions, the respiratory quotient is assumed to be 0.8. Thus the quantity of O2 taken up exceeds the quantity of CO2 that is released in the alveoli. The partial pressures of O2, CO2, and N2 from ambient air to the alveolus at sea level are shown in
Table 23.1
A similar approach can be used to calculate the estimated PACO2. The fraction of CO2 in the alveolus is a function of the rate of CO2 production by the cells during metabolism and the rate at which the CO2 is eliminated from the alveolus. This process of elimination of CO2 is known as alveolar ventilation. The relationship between CO2 production and alveolar ventilation is defined by the alveolar carbon dioxide equation:
Equation 23.14 where V̇ CO2 is the rate of CO2 production by the body, V̇ A is alveolar ventilation per minute, and FACO2 is the fraction of CO2 in dry alveolar gas. This relationship demonstrates that the rate of elimination of CO2 from the alveolus is related to alveolar ventilation and to the fraction of CO2 in the alveolus. Like the partial pressure of any other gas (see
Eq. 23.8 ), PACO2 is defined by the following:
Equation 23.15
Substituting for FACO2 in the previous equation yields the following relationship:
Equation 23.16
This equation demonstrates several important relationships. First, there is an inverse relationship between the partial pressure of CO2 in the alveolus (PACO2) and alveolar ventilation per minute (V̇ A), regardless of the exhaled CO2. Specifically, if ventilation is doubled, PACO2 decreases by 50%. Conversely, if ventilation is decreased by half, the PACO2 doubles. Second, at a constant alveolar ventilation per minute (V̇ A), doubling of the metabolic production of CO2 (V̇ CO2) causes the PACO2 to double. The relationship between V̇ A and PACO2 is depicted in
Fig. 23.1
In normal lungs, PaCO2 is tightly regulated and maintained at 40 ± 2 mm Hg. Increases or decreases in PaCO2, particularly when associated with changes in arterial pH, have profound effects on cell function, including enzyme and protein activity. Specialized chemoreceptors monitor PaCO2 in the brainstem ( ), and exhaled minute ventilation (
Eq. 23.1 ) varies in accordance with the level of PaCO2.